Filter, duplexer and communication module

ABSTRACT

According to an aspect of the invention, each of a filter, a duplexer and a communication module includes a plurality of series resonators connected to a signal line in series and a plurality of parallel resonators connected to the signal line in parallel. At least two of the plurality of parallel resonators are connected to the signal line between two of the plurality of series resonators in parallel, inductors are respectively connected to the at least two parallel resonators, and the inductors have different inductances from each other.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority of theprior Japanese Patent Application No. 2009-15477 filed on Jan. 27, 2009.The entire contents of which is incorporated herein by reference.

FIELD

The disclosure of the present application relates to a filter. Thedisclosure of the present invention relates to a duplexer and acommunication module including the filter.

BACKGROUND

Due to a rapid proliferation of wireless devices represented by mobilephone terminals, demands for high-frequency filters have been increasingat a rapid pace. In particular, there are strong demands for small-sizeand highly precipitous acoustic wave filters.

As the sophistication of wireless systems has advanced at a rapid pacein recent years, specific demands for high-frequency filters have becomehighly complex. For example, it is preferable that a transmission filterand a reception filter included in a duplexer can achieve a low loss ina passband and high suppression in the opposite band (the band of areception filer with respect to the band of a transmission filter, andvice versa).

Generally, filters incorporated in a mobile phone terminal or the likeinclude, in many cases, a plurality of resonators that are connected toeach other so as to ensure a broad passband. For example, JapaneseLaid-open Patent Publication No. 2004-15397 discloses an example of aladder filer.

However, with the above conventional configuration, when the number ofstages in the ladder filter is increased to increase the number ofattenuation poles in suppression bands, a loss in the passband alsoincreases.

Further, with the configuration of connecting inductors to parallelresonators, it is difficult to place attenuation poles in frequencybands where suppression is most needed.

SUMMARY

According to an aspect of the invention, a filter includes a pluralityof series resonators connected to a signal line in series and aplurality of parallel resonators connected to the signal line inparallel. At least two of the plurality of parallel resonators areconnected to the signal line between two of the plurality of seriesresonators in parallel, inductors are respectively connected to the atleast two parallel resonators, and the inductors have differentinductances from each other.

Additional objects and advantages of the invention (embodiment) will beset forth in part in the description which follows, and in part will beobvious from the description, or may be learned by practice of theinvention. The object and advantages of the invention will be realizedand attained by means of the elements and combinations particularlypointed out in the appended claims. It is to be understood that both theforegoing general description and the following detailed description areexemplary and explanatory only and are not restrictive of the invention,as claimed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A is a circuit diagram in which a resonator S is connected to aninput terminal T_(in) and an output terminal T_(out) in series.

FIG. 1B is a circuit diagram in which a resonator P is connected betweenthe input terminal T_(in) and the output terminal T_(out) in parallel.

FIG. 1C is a graph illustrating the frequency characteristics of theresonators S and P.

FIG. 2A is a circuit diagram in which the resonator S depicted in FIG.1A is placed in the series arm and the resonator P depicted in FIG. 1Bis placed in the parallel arm.

FIG. 2B is a graph illustrating the frequency characteristics of thefilter depicted in FIG. 2A.

FIGS. 3A and 3B are circuit diagrams each illustrating the configurationof a ladder filter.

FIG. 4A is a circuit diagram illustrating a filter in which inductancesare respectively connected to parallel resonators.

FIG. 4B is a graph illustrating the frequency characteristics of thefilter depicted in FIG. 4A.

FIG. 5A is a circuit diagram illustrating a filter in which inductancesare respectively connected to parallel resonators.

FIG. 5B is a graph illustrating the frequency characteristics of thefilter depicted in FIG. 4A.

FIGS. 6A to 6C are circuit diagrams each illustrating a modified exampleof a conventional filter.

FIG. 7 is a graph illustrating the frequency characteristics of thefilter depicted in FIG. 6A.

FIG. 8 is a circuit diagram illustrating a first configuration of afilter in one embodiment of the invention.

FIG. 9 is a circuit diagram illustrating a second configuration of thefilter in one embodiment of the invention.

FIGS. 10A to 10C are circuit diagrams each illustrating a modifiedexample of the filter in one embodiment of the invention.

FIG. 11A is a circuit diagram illustrating the conventional filter (thefilter depicted in FIG. 6A) to which a circuit constant is added.

FIG. 11B is a circuit diagram illustrating the filter in one embodimentof the invention (the filter depicted in FIG. 10C) to which a circuitconstant is added.

FIG. 12 is a graph illustrating the frequency characteristics of thefilter depicted in FIG. 11B.

FIG. 13 is a graph illustrating the frequency characteristics in FIG.11A and those in FIG. 12.

FIG. 14A is an enlarged view of the Z portion in FIG. 13.

FIG. 14B is an enlarged view of the Y portion in FIG. 14A.

FIG. 15A is a schematic diagram illustrating an example layout of an SAW(Surface Acoustic Wave) filter chip adopting the filter in oneembodiment of the invention.

FIG. 15B is a plan view illustrating a printed wiring board on which thefilter chip depicted in FIG. 15A can be mounted.

FIG. 16A is a schematic diagram illustrating an example layout of anFBAR (Film Bulk Acoustic Resonator) filter chip adopting the filter inone embodiment of the invention.

FIG. 16B is a plan view illustrating a printed wiring board on which thefilter chip depicted in FIG. 16A can be mounted.

FIG. 17 is a block diagram illustrating a duplexer including the filterin one embodiment of the invention.

FIG. 18 is a block diagram illustrating an RF module.

FIG. 19 is a circuit diagram illustrating a lattice filter.

DESCRIPTION OF EMBODIMENT(S)

In one embodiment of the invention, a filter includes a plurality ofseries resonators connected to a signal line in series and a pluralityof parallel resonators connected to the signal line in parallel. Atleast two of the plurality of parallel resonators are connected to thesignal line between two of the plurality of series resonators inparallel, inductors are respectively connected to the at least twoparallel resonators, and the inductors have different inductances fromeach other.

The filter in one embodiment of the invention, having the basicstructure as described above, can be configured as follows.

In one embodiment of the invention, the plurality of series resonatorsand the plurality of parallel resonators may be connected to each otherin a ladder shape.

In one embodiment of the invention, the plurality of series resonatorsand the plurality of parallel resonators may be connected to each otherin a lattice shape.

In one embodiment of the invention, the plurality of parallel resonatorsincluding the at least two parallel resonators to which the inductorshaving different inductances from each other are respectively connectedmay be grounded through one common inductor.

In one embodiment of the invention, the common inductor may beincorporated in a package or a printed wiring board.

In one embodiment of the invention, at least one of the series resonatorand the parallel resonator is a surface acoustic wave element.

In one embodiment of the invention, at least one of the series resonatorand the parallel resonator is a film bulk acoustic resonator.

In one embodiment of the invention, at least one of the series resonatorand the parallel resonator is a bulk wave element.

According to an aspect of the invention, a duplexer includes atransmission filter and a reception filter. At least one of thetransmission filter and the reception filter includes a plurality ofseries resonators connected to a signal line in series and a pluralityof parallel resonators connected to the signal line in parallel, atleast two of the plurality of parallel resonators are connected to thesignal line between two of the plurality of series resonators inparallel, inductors are respectively connected to the at least twoparallel resonators, and the inductors have different inductances fromeach other.

According to an aspect of the invention, a communication module includesa transmission filter and a reception filter. At least one of thetransmission filter and the reception filter includes a plurality ofseries resonators connected to a signal line in series and a pluralityof parallel resonators connected to the signal line in parallel, atleast two parallel resonators are connected to the signal line betweentwo of the plurality of series resonators in parallel, inductors arerespectively connected to the at least two of the plurality of parallelresonators, and the inductors have different inductances from eachother.

Embodiment [1. Principles of Filter, etc.]

As described above, there are demands for filters to achieve a lowerloss in a passband and higher suppression in a suppression band.Moreover, in recent years, high suppression of harmonics (a secondharmonic, a third harmonic) in a passband is also demanded in some casesso as to avoid cross modulation. Further, in order to avoid interferencewith wireless systems such as a wireless LAN (Local Area Network) andBluetooth (registered trademark), there are also demands for suppressionin the frequency bands that are used by these wireless networks.

As a way to achieve high-frequency filters with low loss, ladder filtershave been widely used. A ladder filter is a high-frequency filter formedby connecting two resonators having different resonance frequencies fromeach other in a ladder shape. The principles of the filter will bedescribed with reference to FIGS. 1A to 1C.

FIG. 1A illustrates a circuit in which a resonator S is connected to aninput terminal T_(in) and an output terminal T_(out) in series. FIG. 1Billustrates a circuit in which a resonator P is connected in parellelbetween the input terminal T_(in) and the output terminal T_(out). Theresonator S depicted in FIG. 1A has a resonance frequency f_(rs) and anantiresonance frequency f_(as). The resonator P depicted in FIG. 1B hasa resonance frequency f_(rp) and an antiresonance frequency f_(ap). FIG.1C is a graph illustrating the resonance characteristics of theresonators S and P.

FIG. 2A is a circuit diagram illustrating a circuit in which theresonator S depicted in FIG. 1A is placed in the series arm and theresonator P depicted in FIG. 2B is placed in the parallel arm. When theantiresonance frequency f_(ap) of the resonator P and the resonancefrequency f_(rs) of the resonator S in FIG. 2A are substantially equalto each other, the filer characteristics illustrated in FIG. 2B can beachieved. In other words, it is possible to achieve a bandpass filerwith a passband that is between the resonance frequency f_(rp) of theresonator P and the antiresonance frequency f_(as) of the resonator Sand an attenuation band that is lower than the resonance frequencyf_(rp) and higher than the antiresonance frequency f_(as).

Generally, in filters incorporated in mobile phone terminals, aplurality of sets of the resonators depicted in FIG. 2A are connected toeach other in many cases. Examples of a filter in which a plurality ofresonators are connected to each other include a ladder filter in whicha plurality of pairs of ladder circuits are connected to each other.

FIG. 3A illustrates an example of the ladder filter. In the ladderfilter, when connecting a plurality of resonators to each other, theadjacent ladder circuits are mirror reversed from one another and areconnected to each other in order to avoid a reflection among stages.Hereinafter, resonators connected to the series arm are referred to asseries resonators and resonators connected to the parallel arm arereferred to as parallel resonators.

In a case where a plurality of resonators are connected to each other asin FIG. 3A, a portion in the series arm where similar series resonatorsare connected and portions in the parallel arm where similar parallelresonators are connected appear, the portions of which are enclosed by adashed line. Since these similar resonators can be combined into asingle resonator in terms of capacity as depicted in FIG. 3B, it ispossible to reduce the chip size. It should be noted that JapaneseLaid-open Patent Publication No. 2004-15397 discloses an example of theladder filter depicted in FIG. 3B.

To achieve high suppression of harmonics (second harmonic, thirdharmonic) in a passband as described above, it has been proposed toconnect inductors to parallel resonators. Since resonators function as acapacity in frequency bands outside the band in which they resonate,parallel resonators to which inductors are respectively connectedfunction as LC resonators in frequency bands outside the band in whichthey resonate.

FIG. 4A is a diagram illustrating a filter in which inductors areconnected to parallel resonators. When inductors L1 and L2 respectivelyhaving inductances H1 and H2 are connected to two parallel resonatorsP11 and P12 each having a capacity Cp, attenuation poles arerespectively formed at frequencies f1 and f2 outside the passband, ascan be seen from FIG. 4B. The frequencies f1 and f2 at which theattenuation poles are formed can be calculated on the basis of thefollowing formulas. In the formulas, Cp denotes the capacity of eachparallel resonator.

f1=1/{2π(H1·Cp)^(1/2)}

f2=1/{2π(H2·Cp)^(1/2)}

By adding more parallel resonators and connecting inductors respectivelyto the added parallel resonators in series, it is possible to increasethe number of attenuation poles.

In contrast, when an inductor L3 is connected to the both parallelresonators P11 and P12 as depicted in FIG. 5A, an attenuation pole isgenerated at a frequency f3 outside the passband, as can be seen fromFIG. 5B.

The filter depicted in FIG. 4A can be modified in several ways andexamples thereof are respectively depicted in FIGS. 6A to 6C. In eithercase, two attenuation poles are generated in a high frequency band ofthe passband due to the inductors and the circuit of each examplebecomes equivalent to that in FIG. 4A.

Here, when the number of stages in the ladder filter is increased toincrease the number of the attenuation poles, a loss in the passbandalso increases. Further, with the configuration of connecting inductorsto parallel resonators, it is difficult to place attenuation poles inthe frequency bands where suppression is most needed.

For example, FIG. 7 is a graph illustrating the frequencycharacteristics of the filter depicted in FIG. 6A. In the frequencycharacteristics of the filter depicted in FIG. 6A, two attenuation polesA and B generated in a high frequency band are respectively placedbetween an Rx band and the Bluetooth band (hereinafter referred to asthe BT band) and between a Tx second harmonic and a Tx third harmonic.It is required that the attenuation poles A and B are placed in thefrequency bands (the BT band, Tx second harmonic band and Tx thirdharmonic band) where suppression is most needed.

In the following embodiment, not only that attenuation poles can beplaced at appropriate frequency positions in a high frequency band butalso a filter with low loss can be provided.

[2. Configuration of Filter]

FIG. 8 is a circuit diagram illustrating a first configuration of thefilter according to the present embodiment. In the filter depicted inFIG. 8, series resonators S11, S12 and S13 are connected to the seriesarm (signal line). Further, parallel resonators P21 and P22 areconnected between the adjacent series resonators S11 and S12 in parallelwith respect to the series arm (signal line). Further, parallelresonators P23 and P24 are connected between the adjacent seriesresonators S12 and S13 in parallel with respect to the series arm.Further, inductors L11, L12, L13 and L14 are connected to the parallelresonators P21, P22, P23 and P24, respectively. The inductors L11 to L14have different inductances (values) from each other.

In the filter depicted in FIG. 8, the parallel resonators that werecombined in one resonator are divided in parallel, and the inductorshaving different inductances from each other are connected to therespective parallel resonators. By configuring the circuit in this way,the number of attenuation poles can be increased without increasing thenumber of stages of the resonators. That is, although the number ofstages of the resonators in the filter depicted in FIG. 8 is 4 similarlyto the filter depicted in FIG. 4A, attenuation poles can be formed atfour places (the number of places at which attenuation poles can beformed by the filter depicted in FIG. 4A is 2).

It should be noted that a pair of one series resonator and one parallelresonator forms one stage in the filter. For example, in the circuitdepicted in FIG. 4A, the series resonator S11 and the parallel resonatorP11, the series resonator S12 and the parallel resonator P11, the seriesresonator S13 and the parallel resonator P12 and the series resonatorS14 and the parallel resonator P12 respectively form pairs, and a filterhaving four stages in total is achieved. In contrast, in the circuitdepicted in FIG. 8, the series resonator S11 and the parallel resonatorP21, the series resonator S12 and the parallel resonator P22, the seriesresonator S13 and the parallel resonator P23 and the series resonatorS14 and the parallel resonator P24 respectively form pairs, and a filterhaving four stages in total is achieved.

FIG. 9 is a circuit diagram illustrating a second configuration of thefilter according to the present embodiment. In the filter depicted inFIG. 9, series resonators S11, S12, and S13 are connected to the seriesarm. Further, parallel resonators P21 and P22 are connected between theseries resonators S11 and S12 in parallel with respect to the seriesarm. Furthermore, a parallel resonator 23 is connected between theseries resonators S12 and S13 in parallel with respect to the seriesarm. Inductors L11, L12 and L13 are connected to the parallel resonatorsP21, P22 and P23, respectively. The inductors L11 to L13 have differentinductances from each other.

In the filter depicted in FIG. 9, the parallel resonators that wereintegrated in one resonator are divided in parallel, and the inductorshaving different inductances from each other are connected to therespective parallel resonators. By configuring the circuit in this way,the number of attenuation poles can be increased without increasing thenumber of stages of the resonators. That is, although the number ofstages of the resonators in the circuit depicted in FIG. 9 is 4similarly to the filter illustrated in FIG. 4A, attenuation poles can beformed at three places (the number of places at which attenuation polescan be formed by the filter depicted in FIG. 4A is 2).

The filter according to the present embodiment can be used as a filtercompatible with WCDMA Band II, for example. In WCDMA Band II, the numberof attenuation poles required in a high frequency band is 3 (the BTband, the Tx second harmonic band and the Tx third harmonic band).Therefore, the filter depicted in FIG. 8 is compatible with WCDMA BandII. Further, even if one branch to which the parallel resonator and theinductor are connected is removed from the filter as depicted in FIG. 9,the filter is still compatible with WCDMA Band II.

FIGS. 10A to 10C respectively illustrate modified examples of the filteraccording to the present embodiment. In the filter depicted in FIG. 10Aan inductor L11 is connected to a parallel resonator P21, an inductorL12 is connected to a parallel resonator P22, and an inductor L13 isconnected to the inductors L11 and L12 and to a parallel resonator P23.At this time, the inductance of each inductor is set such that the totalinductance of the inductors L11 and L13, the total inductance of theinductors L12 and L13 and the inductance of the inductor 13 do notbecome equal to each other.

In the filter depicted in FIG. 10B, an inductor L11 is connected to aparallel resonator P21, an inductor L12 is connected to a parallelresonator P23, and an inductor L13 is connected to the inductors L11 andL12 and to a parallel resonator P22. At this time, the inductance ofeach inductor is set such that the total inductance of the inductors L11and L13, the total inductance of the inductors L12 and L13 and theinductance of the inductor L13 do not become equal to each other.

In the filter depicted in FIG. 10C, an inductor L11 is connected to aparallel resonator P21, an inductor L12 is connected to a parallelresonator P24, and an inductor L13 is connected to the inductors L11 andL12 and to parallel resonators P22 and P23. At this time, the inductanceof each inductor is set such that the total inductance of the inductorsL11 and L13, the total inductance of the inductors L12 and L13 and theinductance of the inductor L13 do not become equal to each other.

Hereinafter, the filters depicted in FIGS. 6A and 10C will be compared.

FIG. 11A is a diagram illustrating the filter in FIG. 6A with a circuitconstant being added. FIG. 11B is a diagram illustrating the filteraccording to the present embodiment (the filter depicted in FIG. 10C)with a circuit constant being added. For example, the circuit constantin FIG. 11B is set such that required suppression becomes 25 dB at theTx second harmonic (3760 MHz), 25 dB at the Tx third harmonic (5640 MHz)and 30 dB in the BT band (2400 to 2500 MHz).

FIG. 12 is a graph illustrating the frequency characteristics of thefilter with the circuit constant in FIG. 11B. As can be seen from FIG.12, with the filter according to the present embodiment, not only thatattenuation poles can be formed at three places, but also the respectiveattenuation poles can be placed in the frequency bands where suppressionis needed. That is, an attenuation pole A can be placed near the BTband, an attenuation pole B can be placed near the Tx second harmonicband (hereinafter referred to as the 2tx band) and an attenuation pole Ccan be placed near the Tx third harmonic band (hereinafter referred toas the 3Tx band).

FIG. 13 is a graph illustrating the frequency characteristicsillustrated in FIG. 12 (indicated by a solid line) and those illustratedin FIG. 11A (indicated by a dashed line). As can be seen from FIG. 13,with the filter depicted in FIG. 11A an attenuation pole D is placedbetween the RX band and the BT band and an attenuation pole E is placedbetween the 2Tx band and the 3Tx band. Therefore, sufficient suppressioncannot be achieved in the BT band, the 2Tx band and the 3Tx band, wheresuppression is most needed. In contrast, with the filter in FIG. 11B,since the attenuation pole A is placed near the BT band, the attenuationpole B is placed near the 2Tx band and the attenuation pole C is placednear the 3Tx band, the frequency bands where suppression is most neededcan be suppressed.

FIG. 14A is an enlarged view illustrating the passband (Z portion) inFIG. 13. FIG. 14B is an enlarged view illustrating the Y portion in FIG.14A. In FIGS. 14A and 14B, the solid line indicates the frequencycharacteristics of the filter depicted in FIG. 11B and the dashed lineindicates the frequency characteristics of the filter depicted in FIG.11A. As can be seen from FIGS. 14A and 14B, the desired frequency bandsin a high frequency band are suppressed by the attenuation poles. Thus,the attenuation of the ladder filter itself can be set to a small value.As a result, it is possible to significantly improve a loss in thepassband.

FIG. 15A is a diagram illustrating a layout example of an SAW (SurfaceAcoustic Wave) filter chip adopting the filter according to the presentembodiment. FIG. 15B is a plan view illustrating a printed wiring boardto which the surface acoustic wave filter chip depicted in FIG. 15A canbe flip-bonded. The filter chip depicted in FIG. 15A is an example ofchip layout of the filter depicted in FIG. 10C. The filter chip depictedin FIG. 15A is an example of a transmission filter chip mounted on acommunication device or the like. In FIG. 15A, a bump electrode 11 isconnected to an antenna terminal Ant depicted in FIG. 15B. A bumpelectrode 12 is connected to a Tx terminal depicted in FIG. 15B and isconnected to a transmission circuit or a power amplifier. A bumpelectrode 13 is connected to an inductor L11 depicted in FIG. 15B. Abump electrode 14 is connected to an inductor L12 depicted in FIG. 15B.A bump electrode 15 is connected to an inductor L13 depicted in FIG.15B. The bump electrodes 11 to 15 are connected to each otherelectrically through a conductor pattern 16. The conductor pattern 16that connects the bump electrodes to each other includes resonators. Theinductors L11, L12 and L13 depicted in FIG. 15B are spiral coils.

In FIG. 15A, resonators C_(s11) and C_(s12) correspond to the seriesresonator S11 in FIG. 10C. Resonators C_(s21) to C_(s23) correspond tothe series resonator S12 in FIG. 10C. Resonators C_(s31) to C_(s33)correspond to the series resonator S13 in FIG. 10C. Resonators C_(p11)and C_(p12) correspond to the parallel resonator P21 in FIG. 10C.Resonators C_(p21) and C_(p22) correspond to the parallel resonator P22in FIG. 10C. A resonator C_(p3) corresponds to the parallel resonatorP23 in FIG. 10C. A resonator C_(p4) corresponds to the parallelresonator P24 in FIG. 10C.

FIG. 16A is a diagram illustrating a layout example of an FBAR (filmbulk acoustic resonator) filter chip adopting the filter according tothe present embodiment. FIG. 16B is a plan view illustrating a printedwiring board on which the filter chip depicted in FIG. 16A can bemounted. The filter chip depicted in FIG. 16A is an example of chiplayout of the filter illustrated in FIG. 10C. The filter chip depictedin FIG. 16A is an example of a transmission filter chip mounted on acommunication device or the like. In FIG. 16A, a bump electrode 21 isconnected to an antenna terminal Ant depicted in FIG. 16B. A bumpelectrode 22 is connected to a Tx terminal depicted in FIG. 16B and isconnected to a transmission circuit or a power amplifier. A bumpelectrode 23 is connected to an inductor L11. A bump electrode 24 isconnected to an inductor L12. A bump electrode 25 is connected to aninductor L13. The bump electrodes 21 to 25 are connected to each otherelectrically through a conductor pattern 26. The conductor pattern 26that connects the bump electrodes to each other includes resonators. Theinductors L11, L12 and L13 depicted in FIG. 16B are spiral coils.

In FIG. 16A, a resonator C_(s1) corresponds to the series resonator S11in FIG. 10C. A resonator C_(s2) corresponds to the series resonator S12in FIG. 10C. Resonators C_(s11) and C_(s12) correspond to the seriesresonator S13 in FIG. 10C. A resonator C_(p1) corresponds to theparallel resonator P21 in FIG. 10C. A resonator C_(p2) corresponds tothe parallel resonator P22 in FIG. 10C. A resonator C_(p3) correspondsto the parallel resonator P23 in FIG. 10C. A resonator C_(p4)corresponds to the parallel resonator P24 in FIG. 10C.

[3. Configuration of Duplexer]

FIG. 17 is a block diagram illustrating a duplexer including the filteraccording to the present embodiment. As can be seen from FIG. 17, aduplexer 102 includes a reception filter 103 and a transmission filter104. The duplexer 102 filters, with the reception filter 103, areception signal inputted thereto through an antenna 101 and extracts areception signal Rx in a desired frequency band. Further, the duplexer102 filters an input transmission signal Tx with the transmission filter104 and outputs the signal through the antenna 101.

The transmission filter 104 may have the circuit configuration depictedin FIGS. 8, 9, 10A, 10B or 10C. As a result, it is possible to set thetransmission filter 104 to comply with the WCDMA Band II standard. As anexample, the passband of the transmission filter 104 can be set to 1850MHz to 1910 MHz. The opposite band (the passband of the receptionfilter) is 1930 MHz to 1990 MHz. In this case, Tx second harmonic can beset to 3760 MHz and Tx third harmonic can be set to 5640 MHz. Further,in order to prevent interference with a system that complies theBluetooth standard, it is possible to suppress the band from 2400 to2500 MHz.

[4. Effects of Embodiment, etc.]

According to the present embodiment, by connecting inductors havingdifferent inductances from each other to parallel resonators in a ladderfilter, the number of attenuation poles can be increased withoutsignificantly increasing the number of stages in the filter.Furthermore, since the attenuation poles can be placed in desiredfrequency bands, a filter that can suppress the desired frequency bandscan be achieved. Moreover, suppression in the passband can be reduced.

In order to clarify the comparison, the capacity ratio of all of theresonators used in the filter in the present embodiment has been fixedto “16”, assuming the case of a surface acoustic wave device. Further,all of the resonance frequencies of the plurality of series resonatorsincluded the filter are set to the same frequency. Further, all of theresonance frequencies of the plurality of parallel resonators includedthe filter are set to the same frequency. It should be noted that whenactually designing a circuit on the basis of the filter according to thepresent embodiment, the capacity ratio and the resonance frequency ofeach resonator may be set arbitrarily.

Although the surface acoustic wave device has been described in thepresent embodiment as an example of implementing the filer, the filtercan be applied also to an FBAR filter, an SMR (solid mounted resonator)filter and other types of ceramic filters.

The filter according to the present embodiment can be applied not onlyto a duplexer, but also to an RF module such as a duplexer bank moduleformed by integrating a plurality of duplexers as a module or a duplexeramplifier module formed by integrating an amplifier and a duplexer as amodule.

FIG. 18 is a block diagram illustrating an RF module including aduplexer bank module. As can be seen from FIG. 18, the RF moduleincludes a switch module 202, a duplexer bank module 203 and anamplifier module 204. The switch module 202 is connected to antennas 201a and 201 b and to duplexers in the duplexer bank module 203. Theduplexer bank module 203 includes a plurality of duplexers 203 a, 203 b,203 c . . . . The filter according to the present embodiment can be usedas a reception filter and/or a transmission filter in each of theduplexers 203 a, 203 b, 203 c . . . in the duplexer bank module 203.

Although the ladder filter has been described in the present embodimentas an example of the filter, similar effects can be achieved by alattice filter depicted in FIG. 19 in which resonators are connected ina lattice shape. The lattice filter includes two signal lines and tielines connected between the signal lines. Series resonators areconnected to the signal lines and parallel resonators are connected tothe tie lines. At least two of the parallel resonators are connectedbetween the signal lines in parallel. By connecting inductors to the twoparallel resonators in series and implementing the inductors usingelements having different inductances from each other, effects similarto those of the present embodiment can be achieved.

All examples and conditional language recited herein are intended forpedagogical purposes to aid the reader in understanding the principlesof the invention and the concepts contributed by the inventor tofurthering the art, and are to be construed as being without limitationto such specifically recited examples and conditions, nor does theorganization of such examples in the specification relate to a showingof the superiority and inferiority of the invention. Although theembodiment(s) of the present invention(s) has (have) been described indetail, it should be understood that the various changes, substitutions,and alterations could be made hereto without departing from the spritand scope of the invention.

1. A filter comprising a plurality of series resonators connected to asignal line in series and a plurality of parallel resonators connectedto the signal line in parallel, wherein at least two of the plurality ofparallel resonators are connected to the signal line between two of theplurality of series resonators in parallel, inductors are respectivelyconnected to the at least two parallel resonators, and the inductorshave different inductances from each other.
 2. The filter according toclaim 1, wherein the plurality of series resonators and the plurality ofparallel resonators are connected to each other in a ladder shape. 3.The filter according to claim 1, wherein the plurality of seriesresonators and the plurality of parallel resonators are connected toeach other in a lattice shape.
 4. The filter according to claim 1,wherein the plurality of parallel resonators including the at least twoparallel resonators to which the inductors having different inductancesfrom each other are respectively connected are grounded through onecommon inductor.
 5. The filter according to claim 2, wherein theplurality of parallel resonators including the at least two parallelresonators to which the inductors having different inductances from eachother are respectively connected are grounded through one commoninductor.
 6. The filter according to claim 4, wherein the commoninductor is incorporated in a package or a printed wiring board.
 7. Thefilter according to claim 1, wherein at least one of the seriesresonator and the parallel resonator is a surface acoustic wave element.8. The filter according to claim 1, wherein at least one of the seriesresonator and the parallel resonator is a film bulk acoustic resonator.9. The filter according to claim 1, wherein at least one of the seriesresonator and the parallel resonator is a bulk wave element.
 10. Aduplexer comprising a transmission filter and a reception filter,wherein at least one of the transmission filter and the reception filterincludes a plurality of series resonators connected to a signal line inseries and a plurality of parallel resonators connected to the signalline in parallel, wherein at least two of the plurality of parallelresonators are connected to the signal line between two of the pluralityof series resonators in parallel, inductors are respectively connectedto the at least two parallel resonators, and the inductors havedifferent inductances from each other.
 11. A communication modulecomprising a transmission filter and a reception filter, wherein atleast one of the transmission filter and the reception filter includes aplurality of series resonators connected to a signal line in series anda plurality of parallel resonators connected to the signal line inparallel, wherein at least two of the plurality parallel resonators areconnected to the signal line between two of the plurality of seriesresonators in parallel, inductors are respectively connected to the atleast two parallel resonators, and the inductors have differentinductances from each other.